Preparation Of Tung Oil-based Toughening Agents And Study On Properties Of Modified Epoxy Resin

Diglycidyl ether of bisphenol A(DGEBA)is an important thermosetting resin,due to the excellent performances of its thermosets in mechanics,thermodynamics,and high temperature and chemical resistances,it is wildly used in the fields like machinery,aerospace,and electronic packaging.However,the rigid structure of DGEBA results in high brittleness and poor impact resistance of its curing products,which are structural defects and cannot be solved intrinsically.Thus,it is necessary to form composite materials by introducing toughening agents to the curing systems to deal with the intrinsic brittle problem.Bio-based materials show advantages in terms of renewability of feedstock,environmental friendliness,low price,and so on.Especially the inherent long carbon chain structure of plant oil,which has the structural basis of toughening epoxy curing system,becomes one of the main research directions of modified epoxy resin.In this study,the key technology for the preparation of toughening agent was developed by using tung oil,a Chinese specialty,as raw material,exploring the relationship between the flexible structure of tung oil and the mechanical properties of epoxy curing products,analysing the influence of the dosage of tung oil based toughening agent on the performance of epoxy curing system,finding out the toughening mechanism of tung oil based toughening agents with different structures,and hence laying a theoretical foundation for the further industrial application of tung oil based toughening agent.Methyl ester of tung oil fatty acid(METOFA)and tung oil fatty acid(TOFA)were synthesized from tung oil,and then random carboxylic copolymer(CTMA)was prepared by free radical polymerization among TOFA,METOFA and acrylonitrile.CTMA was added into the DGEBA/polyetheramine D230 curing system with different mass ratios.The mechanical test results show that CTMA is able to improve the toughness of epoxy curing system effectively,raising the elongation at break from 5.6%to 104.5%,the tensile toughness from 2.94 MJ/m~3 to14.29 MJ/m~3,and the impact strength from 13.62 k J/m~2 to 27.00 k J/m~2.Due to the good compatibility,no phase separation can be observed by scanning electron microscope(SEM).The curing behavior of samples investigated via differential scanning calorimeter(DSC)indicates that the addition of CTMA can catalyze the curing reaction.However,the modified samples exhibit lower glass transition temperature(Tg),tensile strength,tensile modulus and thermal stability.To avoid the great decrement of mechanical strength during toughening process,Diels-Alder reaction between METOFA and maleic anhydride was used to shorten the length of the fatty chain and produce tung-maleic tricarboxylic acid(TMTA),which reacted with 1,4-butanediol diglycidyl ether further to obtain the tung oil-based hyperbranched polyester(TOHBP).After TOHBP being added into the epoxy curing system,impact strength is improved from 20.07 k J/m~2 to 62.79 k J/m~2,and elongation at break and tensile toughness are enhanced by45%and 56%respectively.Besides,the incorporation of TOHBP contributes to a higher cross-linking density of thermosets,increasing tensile strength from 67.97 MPa to 73.01 MPa,tensile modulus from 2610 MPa to 3116 MPa,and Tg from 81.48°C to 86.62°C.DSC results illustrate that TOHBP may accelerate curing reaction,and no phase separation can be observed in SEM images,indicating the homogeneous curing system.On the other hand,thermal stability of the toughened epoxy curing system decreases with the increment of TOHBP.The adduct of tung oil,acrylic acid and acrylonitrile(TAA)was synthesized by a two-step Diels-Alder reaction.This toughening agent contains flexible chains from tung oil,and rigid ring structure,which is similar to the one in TOHBP.Hence,toughening epoxy curing system via TAA is able to improve the toughness and maintain high mechanical strength simultaneously.The mechanical test results show that the elongation at break,tensile toughness and impact strength of the modified epoxy curing system are 25%,7.6 MJ/m~3 and 76.7 k J/m~2 respectively,which are 339%,145%and 262%higher than that of the pure epoxy curing system,and the tensile strength of the sample with the addition of 30 wt%can still be kept at 38 MPa.In adition,TAA is able to improve thermal stability,shifting the temperatures corresponding to the initial decomposition(Ti)and maximum decomposition rate(Tm)to a higher range.Simultaneously,DSC results exhibit that TAA has no significant effect on the onset curing temperature(To),while it leads to higher peak curing temperature(Tp).Modified epoxy curing systems are homogeneous,and Tg values decrease slightly.The synthesized three toughening agents with different structures show different toughening ability.CTMA,which has a large amount of flexible chains,indicates promising toughening effect on elongation at break and tensile toughness,while TOHBP,containing shorter fat chains and hyperbranched structure,is more conducive to improving impact strength and mechanical strength.TAA exhibits the advantages of both CTMA's and TOHBP's,showing excellent overall toughening effect,and significantly improving elongation at break,tensile toughness and impact strength simultaneously.Comparing with other three glyceride-based toughening agents in the literature,the three toughening agents in this study exhibit excellent toughening effects,and industrial production can be carried out by optimizing the preparation process later,which can alleviate the market demand for toughening agents to some extent.In addition,the raw material in this study is tung oil,a natural product,which is renewable.Therefore,this study can not only realize the high-quality utilization of natural products,but also alleviate the dependence of chemical industry on petroleum-based materials to a certain degree.